Active research and development of cognitive radio systems that rely on detection of unused frequencies have been ongoing in recent years. Secondary utilization of frequencies requires that a primary user be not interfered with. To that end, spectrum sensing is performed, such that secondary utilization of a frequency must be discontinued immediately once communication by a primary user is detected.
When implementing cognitive radio in a wireless communication device that moves at high-speed, for instance in an in-vehicle wireless device, the usable frequencies vary, both temporarily and spatially, at any given time. Preferably, therefore, sensing is performed constantly, in order to protect the primary user reliably.
In a case where communication by a secondary user is being performed, a problem arises in that communication by the primary user cannot be detected on account of the near-far problem. Therefore, a method has been proposed that involves providing quiet periods (QP) at which none of the secondary users communicates, as illustrated in FIG. 4, such that sensing is performed in those periods. Such a method is problematic in that the appearance of a primary user can only be detected in the quiet periods, and also problematic in that communication throughput as a secondary user drops due to presence of the quiet periods.
Methods have also been proposed in which communication as a secondary user and detection of a primary user are performed simultaneously. In Non-patent literature 1, part of a signal of a primary user is held as known information, and secondary utilization is discontinued upon reception of the known information in a primary signal, during communication by a secondary user. In this method, the primary signal must be known beforehand as known information, and hence the method is problematic in that, as a result, the scope of application of the method is limited. Further, the method does not address elimination of the near-far problem derived from transmission signals from an own node in a sensing antenna.